Transform options should error on nonexistant targets

[Ryan Prior]

I learned today that Guix will chug happily along applying a transform to a nonexistent package.

For example, I can run:

guix environment --with-latest=not-exist --ad-hoc which

This shows no warning or errors. I think it would be beneficial to show an error & bail if the target of a transformation is a package that doesn't exist, as this is likely indicative of an error.

What do you think?